(1) Field of the Invention
This invention relates to a method of manufacturing a voltage-nonlinear resistor, and more particularly to a method of manufacturing such a resistor based on zinc oxide.
(2) Description of the Prior Art
Various semiconductor circuit elements are known. One of them is a voltage-nonlinear resistor, generally called "varistor". This element has a nonlinear voltage-current characteristic. That is, its resistance abruptly lowers as the voltages applied to it elevate, thus permitting current to increase sharply. It absorbs an abnormally high voltage or stabilizes voltage and is therefore used in electric circuits of various types.
Typical voltage-nonlinear resistors are an SiC varistor, Si varistor and a selenium varistor. Another typical voltage-nonlinear resistors are cuprous oxide- and zinc oxide-based sintered body varistors. An SiC varistor is formed by sintering SiC particles having a diameter of about 100.mu., using a ceramic binder. The SiC varistor can withstand a relatively high voltage. But it cannot be used as a low voltage element because it cannot be made sufficiently thin. An Si varistor is based on a p-n junction which is formed in a silicon substrate. It functions well as a low voltage element. Its use is, however, limited because its voltage-current characteristic cannot be adjusted freely. A selenium varistor and a cuprous oxide-based sintered body varistor exhibit their voltage-current characteristic at the junction of their surfaces with a metal layer and are disadvantageous in that their voltage-current characteristic cannot be controlled freely as in the Si varistor.
By contrast, ZnO-based sintered body varistors, particularly those containing, an impurities, Bi.sub.2 O.sub.3, CoO and Sb.sub.2 O.sub.3, each in an amount up to 10 mol %, have a voltage-current characteristic curve which is symmetrical with respect to the zero-volt axis. These varistors have a good voltage-current characteristic, which can be controlled by changing their thickness. For this advantageous point, ZnO-based sintered body varistors are attracting much attention. The known varistor of this type is, however, not fully satisfactory. Its voltage-current characteristic much varies particularly in negative direction due to external factors such as impulse current, D.C. load and temperature-humidity cycle.